Mice infected with the JHM strain of mouse hepatitis virus (MHV-JHM) develop acute and chronic diseases of the CNS. In one model, suckling C57Bl/6 (B6) mice are infected intranasally with wild type MHV-JHM and nursed by dams previously immunized with live MHV-JHM. These mice are protected from acute encephalitis but a variable fraction (40-90%) develop hindlimb paralysis at 3-8 weeks p.i. We showed previously that in all cases, mutations could be detected in the immunodominant CD8 T cell epitope recognized in this strain (residues 510-518 of the surface (S) glycoprotein [S510]). During the past funding period, we showed that the presence of a mutation in this epitope resulted in inceased morbidity and mortality and also that these mutations arose in the presence of a polyclonal CD8 T cell response with a narrow range of fine specificities. Although we detected CTL escape mutants in all of our MHV-infected mice with chronic demyelination, they are only occasionally detected in other persistent viral infections, included humans infected with HIV. A main objective of this proposal is to determine why CTL escape mutants are selected commonly in MHV-infected mice. These studies will be extended to determining the potential role that CD4 T cell escape mutants have in MHV pathogenesis. These objectives will be studied in the following specific aims: 1) To determine the role of antibody in the development of CTL escape mutants. One striking feature of our suckling mouse model is that mice with chronic demyelination never mount an anti-MHV antibody response. The goals of this specific aim are to determine why antibody responses are not generated in mice that later develop hindlimb paralysis and to probe whether increases in the titer of anti-MHV antibody protect against selection of CTL escape mutants. 2) To determine why mutations in the subdominant CTL epitope S598 are not selected. The role of immune pressure on selection of CTL escape mutants will be examined. 3) To investigate the biological relevance of CD4 T cell escape mutants. Since CD4 T cell escape mutants have not been isolated from infected mice, reverse genetics will be used to introduce mutations into MHV-specific CD4 T cell epitopes. These mutants will be studied in several models of MHV pathogenesis.